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Provisional Regional PBR values for Scottish seals in 2015
D. Thompson, C.D. Morris, C.D. Duck
Sea Mammal Research Unit, Scottish Oceans Institute, University of St Andrews, St Andrews, KY 16
8LB
Abstract
This document estimates Potential Biological Removal (PBR) values for the grey and harbour seal
“populations” that haul out in each of the seven Seal Management Areas (and three subdivisions) in
Scotland. Sets of possible values are tabulated for each area using the equation in Wade (1998) with
different values of that equation’s recovery factor. A value is suggested for this parameter in each
population, the resulting PBR is highlighted, and a rationale is provided for each suggestion. The PBR
values are calculated using the latest confirmed counts in each management area.
Changes since last year: The ongoing decline in harbour seals numbers in Orkney and the East Coast
has reduced the PBRs in those areas. The increase in grey seal numbers in the Moray Firth is almost
balanced by a reduction in Orkney.
Introduction
Potential Biological Removal is a widely used way of calculating whether current levels of
anthropogenic mortality are consistent with reaching or exceeding a specific target population,
chosen to be the Optimum Sustainable Population (OSP). It is explicitly given, in an amendment to
the US Marine Mammal Protection Act, as the method to be used for assessing anthropogenic
impacts in the waters around that country. The method has been supported by simulations
demonstrating its performance under certain assumptions (Wade 1998). The PBR calculation is based
on population size and intrinsic rate of increase and will therefore produce a PBR even for declining
populations. The formulation includes a recovery factor (FR) which can be altered to take account of
population status. However, FR only varies between 0.1 and 1 so the PBR method allows for small
anthropogenic takes from any population, however much it is depleted or fast it is declining.
Scottish Government uses PBR to estimate permissible anthropogenic takes for each of the seven
seal management regions and uses this information to assess licence applications for seal control and
for other licensable marine activities.
Materials and Methods
The PBR calculation:
PBR = Nmin.(Rmax/2).FR
where:
PBR is a number of animals considered safely removable from the population.
Nmin is a minimum population estimate (usually the 20th percentile of a distribution)
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Rmax is the population growth rate at low densities (by default set 0.12 for pinnipeds), this is
halved to give an estimate of the growth rate at higher populations. This estimate should be
conservative for most populations at their OSP.
FR is a recovery factor, usually in the range 0.1 to 1. Low recovery factors give some
protection from stochastic effects and overestimation of the other parameters. They also
increase the expected equilibrium population size under the PBR.
The approach and calculation is discussed in detail in Wade (1998).
Data used in these calculations
Nmin values used in these calculations are from the most recent summer surveys of each area, for
both species:

Harbour seals:
The surveys took place during the harbour seal moult, when the majority of this species will be
hauled out, so the counts are used directly as values for Nmin. (An alternative approach, closer to
that suggested by Wade (1998), would be to rescale these counts into abundance estimates and take
the 20th centile of the resulting distributions. Results of a recent telemetry study of harbour seal
haul-out behaviour (Lonergan et al., 2012) indicate sex linked differences in haul-out patterns during
the survey period. We do not have any information on the sex ratios of Scottish harbour seal
populations, but the observed patterns suggest that the PBRs would decrease if the populations are
predominantly female.)

Grey seals:
Analysis of telemetry data from 107 grey seals tagged by SMRU between 1998 and 2007 shows that
around 31% were hauled out during the survey windows (Lonergan et al., 2011a). The 20th centile of
the distribution of multipliers from counts to abundances implied by that data is 2.56.
Rmax is set at 0.12, the default value for pinnipeds, since very little information relevant to this
parameter is available for Scottish seals. A lower value could be argued for, on the basis that the
fastest recorded growth rate for the East Anglian harbour seal population has been below 10%
(Lonergan et al. 2007), though that in the Wadden Sea has been consistently growing at slightly over
12% p.a. (Reijnders et al. 2010). Regional pup production estimates for the UK grey seal population
have also had maximum growth rates in the range 5-10% p.a. (Lonergan et al. 2011b). However the
large grey seal population at Sable Island in Canada has grown at nearly 13% p.a. (Bowen et al. 2003).
FR needs to be chosen from the range [0.1, 1]. Estimated PBR values for the entire range of FR values
are presented. A recommended FR value is indicated for each species in each region, together with a
justification for the recommended value.
Areas used in the calculations
Figure 1 and Table 1 shows the boundaries of the Seal Management Areas.
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Table 1: Boundaries of the Seal Management Areas in Scotland.
Seal Management Area
1
2
3
4
5
6
7
South-West Scotland
West Scotland
Western Isles
North Coast & Orkney
Shetland
Moray Firth
East Coast
Area covered
English border to Mull of Kintyre
Mull of Kintyre to Cape Wrath
Western Isles incl. St Kilda, Flannan Isles, North Rona
North Mainland coast & Orkney
Shetland incl. Foula & Fair Isle
Duncansby Head to Fraserburgh
Fraserburgh to English border
Particularly for grey seals, there will probably be substantial movement of animals between these
areas. The division is a pragmatic compromise that attempts to balance: current biological
knowledge; distances between major haul-out sites; environmental conditions; the spatial structure
of existing data; practical constraints on future data collection; and management requirements.
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Results
PBR values for grey and harbour seals for each Seal Management Area. Recommended FR values are highlighted in grey cells.
Table 2. Potential Biological Removal (PBR) values for harbour seals in Scotland by Seal Management Unit for the year 2015.
2007-2013
Seal Management Unit
1 Southwest Scotland
2 West Scotland
3 Western Isles
4 North Coast & Orkney
5 Shetland
6 Moray Firth
7 East Scotland
SCOTLAND TOTAL
PBRs based on recovery factors FR ranging from 0.1 to 1.0
count
Survey years
834
11,057
2,739
1,938
3,039
898
215
(2007)
20,720
Nmin
0.1
selected
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
FR
PBR
45
597
147
104
164
48
11
50
663
164
116
182
53
12
0.7
0.7
0.5
0.1
0.1
0.3
0.1
35
464
82
11
18
16
1
(2007; 2013)
834
11,057
2,739
1,938
3,039
898
215
5
66
16
11
18
5
1
10
132
32
23
36
10
2
15
199
49
34
54
16
3
20
265
65
46
72
21
5
25
331
82
58
91
26
6
30
398
98
69
109
32
7
35
464
115
81
127
37
9
40
530
131
93
145
43
10
(2007-2009; 2011;
2013)
20,720
122
245
370
494
619
743
868
992 1,116 1,240
(2007-2009; 2013)
(2011)
(2013)
(2009)
(2008; 2011; 2013)
627
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Table 3. Potential Biological Removal (PBR) values for grey seals in Scotland by Seal Management Unit for the year 2015.
2007-2013
Seal Management Unit
1 Southwest Scotland
2 West Scotland
3 Western Isles
4 North Coast & Orkney
5 Shetland
6 Moray Firth
7 East Scotland
SCOTLAND TOTAL
count
374
2,700
2,518
8,079
1,536
1,311
1,935
18,453
PBRs based on recovery factors FR ranging from 0.1 to 1.0
Survey years
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
(2007; 2013)
957
6,912
6,446
20,682
3,932
3,356
4,954
5
41
38
124
23
20
29
11
82
77
248
47
40
59
17
124
116
372
70
60
89
22
165
154
496
94
80
118
28
207
193
620
117
100
148
34
248
232
744
141
120
178
40
290
270
868
165
140
208
45
51
57
331 373 414
309 348 386
992 1,116 1,240
188 212 235
161 181 201
237 267 297
(2007-2009; 2011;
2013)
47,240
280
564
848 1,129 1,413 1,697 1,981 2,263 2,548 2,830
(2007)
(2007-2009; 2013)
(2011)
(2013)
(2009)
(2008; 2011; 2013)
Nmin
0.9
selected
1
FR
PBR
1
1
1
1
1
1
1
57
414
386
1,240
235
201
297
2,830
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Rationale for the suggested recovery factors
The original PBR methodology leaves the setting of the recovery factor as a subjective choice for
managers. Factors such as the amount of information available about the population (and in
particular its maximum annual growth rate), recent trends in local abundance, and the connections
to neighbouring populations are relevant to setting this. The main factors affecting the value
suggested for each species in each area are given below:
Harbour seals
1) Shetland, Orkney + North Coast and East Scotland (FR = 0.1)
FR set to minimum because populations are experiencing prolonged declines.
2) Outer Hebrides (FR = 0.5)
Population was undergoing a protracted but gradual decline but the most recent count was close to
the pre-decline numbers. The population is only partly closed being close to the relatively much
larger population in the Western Scotland region, and the Rmax parameter is derived from other seal
populations.
4) Western Scotland (FR = 0.7)
The population is largely closed, likely to have limited interchange with much smaller adjacent
populations. The population is apparently stable and the intrinsic population growth rate is taken
from other similar populations.
4) Southwest Scotland (FR = 0.7)
The population is apparently stable, is closed to the south and the adjacent population to the north is
apparently stable. The intrinsic population growth rate is taken from other similar populations.
5) Moray Firth (FR = 0.3)
The recent counts for the Moray Firth show large inter annual fluctuations after a period of gradual
decline. The higher counts in some years suggest that this population may be slowly recovering from
the declines that occurred in the years around 2000. The neighbouring Orkney and Tay populations
are continuing to undergo unexplained rapid and catastrophic declines in abundance. Data available
from electronic telemetry tags suggest there is limited movement between these three areas. The
PBR was set at 17 for 2013, permits for 16 harbour seals were granted and 3 were shot. We
therefore, suggest that the FR should be again set to a value of 0.3.
Grey seals
All regions (FR = 1.0)
There has been sustained growth in the numbers of pups born in all areas over the last 30 years, with
some now appearing to be at or close to their carrying capacities (Lonergan et al. 2011b). Available
telemetry data and the differences in the regional patterns of pup production and summer haul-out
counts (Lonergan et al. 2011a), also suggest substantial long-distance movements of individuals.
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References
Bowen, W. D., McMillan, J. & Mohn, R. (2003). Sustained exponential population growth of grey seals
at Sable Island, nova Scotia. ICES Journal of Marine Science, 60, 1265-1274.
Lonergan, M., Duck, C. D., Thompson, D., Mackey, B. L., Cunningham, L. & Boyd, I. L. (2007). Using
sparse survey data to investigate the declining abundance of British harbour seals. Journal of
Zoology, 271, 261-269.
Lonergan, M., Duck, C. D., Thompson, D., Moss, S. & Mcconnell, B. (2011a) British grey seal
(Halichoerus grypus) abundance in 2008: an assessment based on aerial counts and satellite
telemetry. ICES Journal of Marine Science: Journal du Conseil, 68, 2201-2209.
Lonergan, M., Thompson, D., Thomas, L. & Duck, C. (2011b). An Approximate Bayesian Method
Applied to Estimating the Trajectories of Four British Grey Seal (Halichoerus grypus) Populations from
Pup Counts. Journal of Marine Biology, 2011.
Lonergan, M., Duck, C., Moss, S., Morris, C. & Thompson, D. (2012) Rescaling of aerial survey data
with information from small numbers of telemetry tags to estimate the size of a declining harbour
seal population. Aquatic Conservation: Marine and Freshwater Ecosystems, DOI: 10.1002/aqc.2277.
Reijnders, P.J.H., Brasseur, S.M.J.M., Tougaard, S., Siebert, U., Borchardt, T. and Stede, M. (2010).
Population development and status of harbour seals (Phoca vitulina) in the Wadden Sea. NAMMCO
Scientific Publications, 8, 95-106.
Wade, P.R. (1998) Calculating limits to the allowable human-caused mortality of cetaceans and
pinnipeds. Marine Mammal Science 14(1):1:37
Boyd, I.L., Thompson, D. & Lonergan, M. (unpublished) Potential Biological Removal as a method for
setting the impact limits for UK marine mammal populations. Draft briefing paper to 2009 SCOS
meeting.
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Figure 1. Seal management areas in Scotland.
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